Hey everyone, I've designed my own coilover kit using the Bilstein AS2 line of parts as its basis.

Here's a image of the front and rear designs in CAD:



Here are some shots of the machined parts before installation:



Here's a picture of the shock dyno and assembly:



Installed in the car:



Images of the shock dyno graphs:




I went with 700 lb/in front and 375 lb/in rear spring rates (12.5 kg/mm front, 6.7 kg/mm rear). The rear is installed as true rear. I calculated an approximate damper curve for those spring rates and matched the real life builds to those theoretical curves. The Bilstein AS2 shaft I used has an "open" adjuster, so it adjusts both compression and rebound simultaneously (single open hole).

So far I've put about 1k miles on the parts and I'm quite happy so far. The ride is very firm, but not harsh at all, and the handling is definitely sharper with those spring rates. My plan is to turn this car primarily into a track day/weekend mountain car, and this coilover kit is a good step in that direction.

I'm reaching out to the 350z community to see if anyone would be interested in a kit like this... I can order these parts and custom build a kit for anyone. Also, Bilstein shock parts fit inside BC Racing shock bodies, so I can also build these damper curves into an existing BC Racing unit if anyone wants a retrofitted kit.

Feel free to reach out with questions and comments. Thanks for reading!

 

nice job. let us know how they hold up after some track days

 

Do you have any length documentation? As in compressed length to bumpstop vs oem and total droop vs oem? It appears that there is not damper height adjustment, only spring seat adjustment which is fine, but I am curious on where the damper sits in operation and the total sweep lengths vs an oem damper setup.

 

Great question. My basic design was to have the free length of the assembly be 0.5'' shorter than the OEM for both front and rear. My OEM benchmark was the NISMO S Tune kit since that's what I had on the car.

The measured front stroke is 3.93''
The measured rear stroke is 5.5''

The Bilstein shafts can be purchased in various lengths, but my goal is to use a shaft that is, at minimum, capable of the same travel as OEM. Therefore the shafts I designed around are:
5'' front shaft which allows for 4.71'' of stroke
7'' rear shaft with allows for 6.58'' of stroke

The Bilstein threaded bodies can be purchased in various lengths, the practical limit being the shaft length. You must choose a threaded body that can accommodate the chosen shaft stroke.

With those choices made, I designed the mounting to create an assembly that is 0.5'' shorter than OEM. This sounds simple, but I screwed it up and had to remake the lower mount on the front coilover. The rear assembly ended up slightly longer than I expected, but close enough to not have to make new parts. I have V2 designs for all that stuff.

The bump rubbers are really there just to protect the internals of the shock. I don't expect any bump rubber engagement since the spring rates are so much higher than stock. That said, as I test drive these units I'll pay attention to shaft travel and bump rubber engagement. Engagement gaps can be tuned with packers.

The ride height and travel theory I've subscribed to is the 1/3 rd theory. You take the total travel of the shock and multiply by 0.3333. That's the amount of rebound travel you want from curb ride height condition. The other 2/3rds of the shock travel is used for bump. Adhering to that rule works pretty well in my experience. The spring length and preload are chosen in concert to have 2/3rd travel available for bump and 1/3rd available for droop.

If someone wanted this kit to be super short or something, then you can choose the shaft and body length to target a specific ride height. For example, rather than being 0.5'' lower than stock, you might want to be 1'' or 1.5'' lower than stock. That can be achieved by using the appropriate length shock body and shaft.

 

Incredible work! How would you say this setup compares to the S-Tune kit you had before? Is it more or less firm?

I primarily daily my Z on public CA roads which are not the kindest. Overall I'm happy with my Bilstein B6 dampers paired with the factory equipped 07-08 Nismo Coils (different from S-Tune), but I have noticed the rear suspension rebound can be violent when hitting larger bumps or dips at high speeds, which has the potential to unsettle the rear end when cornering or in wet driving conditions. I suppose this is primarily due to the additional travel in the rear suspension, it compresses nicely, but then pops back up hard & fast.

Cheers!
-Icer

 

I liked the S-Tune kit. In my opinion, that level of stiffness would have been about right as the OE setup. Any softer than that, and I think you loose a sort of sportiness that these cars should have from the factory. In my opinion, the OE suspension was just too soft for these cars. I based some of my initial calculations off of how the S-Tune kit is setup. Specifically, the difference between the front and rear ride frequency. I know those engineers would have purposefully designed that setup to maintain a flat ride characteristic with the car.

That's why your comments about your NISMO + B6s are interesting. Your comments sound like a car that is getting into the bump stops in the rear, then as they unload from that compressed state, there's not enough rebound damping to control the body movement. Usually, an OE style shock will have 4x the rebound control vs bump control. I highly suspect that's the case with the B6s, but I've never dynoed those shocks. I did a quick search on the NISMO spring rates, and form what I can see they're not super stiff, so I would imagine the Bilsteins would have no issue controlling that spring rate.

From what I see online, the B6 shocks have 3.86'' of travel: https://www.shockwarehouse.com/produ...43212545327336
Let's say you compress them 0.5'' to install, then they travel another 1'' from installed length to curb ride height, that leaves you with 2.36'' of available stroke. The OEM bump rubber from my S-Tune kit is 2.25'' tall. If you take those numbers as correct, that would mean you're basically always in the bump rubbers. That should feel jittery for small road imperfections, and jumpy for large body movements.

My assumptions could be wrong, but it sounds like you're riding the bump stops. The B8 shock is sorter, which means you'd compress it less when it's being installed. That would leave a little extra room on the shaft before the bump rubber touches. It would be interesting to just uninstall the bump rubber and see if the ride is more comfortable.

For my setup, I love it. It's sharp, sporty, and still nice and flat (doesn't pitch back and forth). The damping level gives me a very predictable sense of what the body of the car will do next. I would say the overall stiffness is just below where it would start to get annoying. My target ride frequency was about 2Hz specifically because going beyond that gets harsh and annoying. I'm pretty happy with how it turned out.

 

Now you got me wondering. I'll need to check under the shaft dust covers to see if the rubber bump stops have been getting hammered or not. To be fair, this behavior only manifests under very specific road conditions, most of the time my Z feels very planted/stable. It's hard to describe the condition, but imagine a section of freeway (like sky ramps or bridges) with low and high spots due to interconnects sagging between support columns. Cresting those high spots at speed and immediately going into a dip makes the rear suspension feel very squirrely.
-Icer

 

Yeah that totally makes sense. Extra large compression movements basically. There's a section of highway I've been using to test that very thing near where I live. Basically, a concrete bridge that's the interchange between two highways. When you go from the concrete back to the asphalt there's a huge divot that causes the car to compress a lot. From the testing I've done so far, this is the maximum damper travel I've observed. And with my setup I'm touching the bump rubber, but I don't think I'm into it a bunch. For what it's worth, I think my setup feels really nice in those big compression events. The body seems to settle quickly. It's certainly not like driving a big heavy SUV, but also not that harsh either.

 

Do you plan on making anything with external reservoirs like the fortune pro 2 or 3 way? I have a track car only with aero and am running the 510s now.

 

I would love to do a project like that for a serious track car. The Bilstein external reservoir and adjuster could easily be adapted to this kit. It's not totally necessary to add adjustment options, but that does add a ton of flexibility, obviously at increased cost and complexity though.

If you wanted to do something like that I'm totally game. I'd start by understanding all the basics of the car... total weight, % front weight, spring rates, target ride heights, other suspension mods. If possible, I'd dyno the FA 510s to see what your baseline setup is really doing. From there, I can calculate basic ride frequency targets and the necessary damping ratios. If there are issues with your ride and handling that you want to address, you can get pretty far with just the basics and a spreadsheet.

Let me know if that's something you're interested in!

 

I like your methods. Nice job man.

Do you have an estimated cost break down? This has my interest piqued.

 

This is a great question. I did a really quick and dirty cost estimate using the front assembly. The rear would be much the same.



What I'm estimating here is just under 2k per corner. There are some HUGE caveats with this estimation however.

Purchased Parts:
Purchased parts are in blue. I'm paying full retail for Bilstein parts from a catalog. Consumables like oil and shims are sort of an estimate, but I think pretty close. I would try to get a break on Bilstein prices from a retailer, but honestly their margins are pretty low so I might be stuck with Bilstein MSRP. That said, Bilstein is top of the line stuff.

Machined Parts:
I didn't pay for my machined parts because they were produced in a quid pro quo deal with someone who owns a machine shop. The estimates I'm using here are from previous experience and quotes I've gotten throughout the years from other projects.

I will say this about machined parts... the price range for machining is absolutely wild. I'm not joking when I say I've had a part quoted at $50 per piece and $500 per piece (same quote, different bidder). My point is that the machined parts could be very inexpensive if the right overseas supplier is sourced. In my experience, the biggest drawback from using machine shops in Asia (mostly China), is the lead times are all over the place. Even if you find the right supplier at the exact right price, you'll likely have to wait a couple months to get your parts.

If you, GreyZ, or someone else was interested in a project like this, I'd try to source machining within North America first. I know a couple machine shop owners that would likely be reasonably priced. From there, if the machined part cost was too high, I'd start to look overseas for small quantity machining, which I've done many times in the past for other projects. Of course, large quantity part runs bring the per piece cost down substantially. However, I don't think I'll be mass producing coilovers anytime soon lol.

The alternative to this is to swap internals from existing coilovers. I know for a fact that Bilstein parts fit inside BC Racing bodies (I've done that project before). In that scenario, no parts are being machined at all, and purchased parts are kept at a bare minimum.

Let me know what you guys all think.